1. Field of the Invention
The present invention relates to a light intensity detector usually incorporated in an optical transmitting apparatus such as an optical modulator, for example.
2. Description of the Prior Art
An optical modulator is utilized to realize optical transmission of information. The optical modulator often includes an optical chip made of a dielectric crystal such as LiNbO3. A light waveguide is formed in the optical chip. So-called electrooptic effect serves to induce a shift of phase of the input light in the light waveguide. This results in changes in the intensity of the input light. The obtained output light from the light waveguide is transmitted to the receiver through a fiber optics. The changes in the intensity of the light are utilized to discriminate binary values of information data.
An electric signal is input into a signal electrode on the optical chip from a signal source so as to shift the phase of the input light. If the biasing voltage for the electric signal suffers from the fluctuation of an operating point, such as a thermal drift, a direct current (DC) drift, or the like, the phase of the input light cannot shift with accuracy. A so-called feedback control is necessarily employed to adjust the biasing voltage for the electric signal at a uniform level. In this case, a photodiode is employed to monitor the intensity of the output light, for example.
As disclosed in Japanese Patent Application Publication 4-263215, a polarization separating circuit is related to the optical chip to monitor the intensity of the output light. A light including TM and TE polarizations is input into the light waveguide on the optical chip. The polarization separating circuit serves to derive the TM and TE polarizations from the light output from the light waveguide. The TM polarization is transmitted to the receiver or a destination through the fiber optics. The TE polarization is received at the photodiode.
The TE polarization is utilized for the detection of the intensity of the output light. As conventionally known, the optical chip provides different modulation efficiencies for the TE and TM polarizations, since the polarization plane of the TE polarization is perpendicular to that of the TM polarization. A shift of phase naturally occurs between the TE and TM polarizations. A difference in the extinction ratio may also be induced between the TE and TM polarizations. Accordingly, the intensity of the TM polarization cannot be monitored with accuracy at the photodiode in the polarization separating circuit of the mentioned publication.